The invention relates to a method for multi-hop data transmission in an ad-hoc network with concealed nodes.
A mobile ad-hoc network (MANET) consists of a quantity of radio devices, which form a temporary network in a dynamic manner, in which every radio device acts as a router for the respective other radio devices and accordingly, no central infrastructure is required.
One important point with MANETs is the co-ordination of access to the jointly-used transmission resource. In each case only one device may transmit, because otherwise collisions can occur, which lead to the loss of information transmitted.
A medium-access-control (MAC) protocol coordinates access and defines how the radio devices can divide the limited transmission resource in an efficient and fair manner. Particularly in the ad-hoc domain, MAC methods, which co-ordinate channel access on the basis of information regarding the channel loading, are extremely-widespread. Methods of this kind are referred to in general by the term carrier-sensing-multiple-access (CSMA). One widely used WLAN method IEEE 802.11 is based on CSMA.
In the case of CSMA methods, all stations wishing to transmit monitor the medium. If the medium is already occupied, the stations wishing to transmit must wait. If the medium is not occupied, the stations wishing to transmit may occupy the medium and transmit. In order to avoid collisions at the end of a transmission, the channel access can be controlled by a random waiting time. Different stations have different waiting times at the end of a transmission, so that one station occupies the medium first. The other stations recognise this through channel monitoring and suspend their transmission request until the end of the current transmission. The medium is regarded as occupied, if the received signal strength is disposed above a given threshold value.
MANETs have a dynamic, random topology, which sometimes changes rapidly. The radio devices communicate directly with their neighbouring radio devices (adjacent nodes) if these are within the range, or otherwise via several radio devices acting as intermediate nodes (multi-hop), which route the information to the target radio device (target node). Such networks are very robust because of the decentralised structure; however, they require independent control and co-ordination by the individual network nodes.
For a multi-hop operation, every radio node provides a routing table with the topology of all participating radio nodes. In the case of proactive routing within a given time raster, these routing tables are updated by means of data exchange with the adjacent radio nodes; in the case of reactive routing, the routing tables are updated only at the times of the user-data exchange with the adjacent radio nodes. If the source node finds the target node in the routing table, the data packet to be transmitted is transmitted from the radio node wishing to transmit via the routing pathway for all known intermediate nodes to the target node.
As shown in FIG. 1, a network blockage can occur, if two nodes, within the range of which the respective, other node is not contained, share an identical node, which is disposed within their range. If the two nodes wish to communicate with their identical node at the same time, their respective signals will collide, and this cannot be prevented by the CSMA method because of the lack of co-ordination between the two nodes. The performance capability of the mobile ad-hoc network is reduced by a network blockage of this kind. Since the two transmitting nodes are each disposed outside the range of the other node, they each represent concealed nodes for the respective, other node.
The identification of concealed nodes of this kind therefore represents an important measure for increasing the performance capacity of the mobile ad-hoc network.
The RTS-CTS method according to FIG. 2 is often used in order to avoid collisions between the concealed nodes. In this context, a transmitter first transmits a short message (RTS) for notification of the transmission. The receiver also responds with a short message (CTS). This is followed by the actual user-data transmission. As a result of the RTS and CTS messages, all stations within the range of the transmitter and receiver are notified about the transmission. During the transmission, the other stations themselves must not transmit in order to avoid collisions.
The transmission of RTS and CTS messages uses transmission capacity, which is therefore not available for the transmission of the user-data packets. Especially with a transmission of small user-data packets, the transmission of RTS and CTS messages has a significant, negative impact. Accordingly, the RTS-CTS method is used only above a given size of user-data packet.
WO/03/079708 A1 discloses a method for identifying a concealed node. In this context, a control message is transmitted from a node wishing to transmit to an adjacent node, in the vicinity of which a node concealed from the node wishing to transmit is disposed, and is finally routed from the adjacent node to the concealed node, which sends an acknowledgement message back to the node wishing to transmit via the adjacent node. This acknowledgement message is used by the node wishing to transmit as an indicator for the existence of a concealed node disposed in the vicinity of the adjacent node.